1. Field of the Invention
The present invention relates to a hydrostatic continuously variable transmission configured by connecting a hydraulic pump and a hydraulic motor via a hydraulic closed circuit and configured to enable variably controlling of the capacity of at least either of the hydraulic pump or the hydraulic motor, shifting the input revolution speed of the hydraulic pump and acquiring the output revolution speed of the hydraulic motor.
2. Description of Background Art
Various configurations of a hydrostatic continuously variable transmission are known. For example, hydrostatic continuously variable transmissions have been proposed and disclosed in JP-A No. H6-42446; JP Patent No. 2920772; JP-A No. H9-100909 and JP-A No. 2005-256979 by the applicants. These hydrostatic continuously variable transmissions disclosed in these patents and applications are each provided with a swash plate type plunger pump, a swash plate type plunger motor and a hydraulic closed circuit that connects a discharge port and a suction port of the swash plate type plunger pump to a suction port and a discharge port of the swash plate type plunger motor. The hydrostatic continuously variable transmission is configured so that a pump swash plate is driven by an engine, a pump cylinder and a motor cylinder are connected and are arranged on an output shaft, the rotation of a motor swash plate is regulated, and an angle of the motor swash plate can be variably adjusted.
A hydrostatic continuously variable transmission configured as described above is known. A clutch valve is provided that connects and cuts off an oil passage on the high pressure side and an oil passage on the low pressure side respectively forming the hydraulic closed circuit. A quantity in which a rotational driving force from the hydraulic pump is transmitted to the hydraulic motor is controlled. A clutch control for cutting off the rotational transmission is executed. For example, in JP-A No. 2005-256979, an automatic clutch using such a clutch valve is disclosed. This clutch valve is provided with a valve spool movably arranged in a spool hole axially extending in the supporting shaft that rotatably supports the hydraulic pump and the hydraulic motor, and connects and cuts off the oil passage on the high pressure side and the oil passage on the low pressure side by axially moving the valve spool. The clutch valve is provided with a spring (energizing means) that energizes the valve spool in a direction of disengagement and a centrifugal governor that generates a force corresponding to input revolution speed. The clutch valve is opened and closed according to a balance among the energizing force by the spring, a governor force and a load depending upon internal pressure (high pressure), and executes control for connecting and cutting off the oil passage on the high pressure side and the oil passage on the low pressure side.
In the above-mentioned clutch valve, the valve spool requires a part that receives an energizing force by the spring and governor force, a part that guides to enable an axial smooth movement in the spool hole and a part that connects and cuts off the oil passage on the high pressure side and the oil passage on the low pressure side according to the axial movement. Thus, the valve spool is formed in an axially long shape. In this case, as high precision is required for the peripheral dimension of a guide part fitted into a guide hole formed in the supporting shaft and guided to be axially moved in the spool hole and the peripheral dimension of a valve part fitted to a part in which the oil passage on the high pressure side and the oil passage on the low pressure side are open in the spool hole for connecting and cutting off the oil passage on the high pressure side and the oil passage on the low pressure side according to the axial movement, the above-mentioned clutch valve has a problem in that the manufacture of the valve spool is difficult and a great deal of manufacturing cost is required. Further, a problem exists wherein the precision is not met. Thus, the operation performance may be deteriorated.